1. Technical Background
The invention relates to a method and an apparatus for scintillation counting of ionizing radiation.
In radionuclide laboratories, in nuclear facilities and in general radiation protection, measurements are regularly conducted, e.g., for establishing radioactive contamination or the dose rate, as well as for activity measuring.
In process measurement technology, such as for measuring fill level, density, or weight per unit area, nuclear measuring processes are widely used as well. The detectors that are utilized are especially scintillation counters, counting tubes and ionization chambers.
2. Prior Art
The publication by KNOLL, entitled “Radiation detection and measurement”, 2nd edition (p. 231 and 237) discloses that ZnS, and specifically ZnS(Ag), is the material of choice for detection of alpha radiation and has a high light output in response to that radiation. In contrast, ZnS(Ag) is apparently not suitable for detection of other types of radiation.
The prior art, which has already been described in U.S. Pat. No. 5,796,108, is thus as follows:
If only alpha radiation is measured, only ZnS(Ag) is used as the scintillator. If only beta radiation is measured, a plastic scintillator is used. However, if alpha and beta radiation are to be measured simultaneously and separately, a “sandwich” scintillator is used. It consists of a flat plastic scintillator with an applied layer of ZnS(Ag), with the latter facing the probe. The thickness of the ZnS(Ag) layer is selected such that preferably all alpha particles are stopped and generate light in the process, which is achieved with a ZnS(Ag) layer thickness of approximately 6 mg/cm2. In all embodiments, a light-proof radiation inlet window, usually a metallized plastic film, is located above the scintillator.
The scintillator may be mounted directly on the inlet window of the photomultiplier. This is not possible, however, with surface-contamination monitors since the scintillators have surface areas of typically 100-200 cm2, whereas the inlet windows of preferred photomultipliers have a diameter of only approximately 25 mm. The photons are thus bundled in this case from the scintillator with the aid of a reflector onto the photocathode.
The output pulses of the photomultiplier pass through a linear amplifier with pulse shaping times of typically 1-20 μs. The discrimination between alpha radiation on one hand, and beta/gamma/X-radiation on the other hand, takes place in the known process with sandwich scintillators based on amplitudes that are sorted by pulse amplitude discriminators into the corresponding channels.
This method has a number of shortcomings:
The sensitivity to low-energy beta radiation is low, since this radiation must first penetrate the ZnS(Ag) layer where it does not generate any signal that can be measured with conventional methods, before reaching, with its residual energy, the plastic scintillator. One also no longer obtains a visible plateau in this case, i.e., no stable operating point in dependence upon the pulse rate as a function of the high voltage.
Plastic scintillators with ZnS(Ag) coating additionally require a special and complex manufacturing process and, consequently, high costs for the detector and thus for the measuring system.